The effect of grain size, alloy composition and turbostratic disorder on the thermal and electrical properties of bismuth telluride-based materials

The thermal conductivities of TiTe2, Bi2Te 3, Sb2Te3, (BixSb1-x) 2Te3, [(Bi2Te3)]m[(TiTe 2)1.36]n and [(BixSb1-x) 2Te3)]m[(TiTe2)1.36] n are reported. The thermal conductivity of Bi2Te3, Sb2Te3, and (Bi0.5Sb0.5) 2Te3 were found to vary linearly with grain size between 10 and 100 nm. The thermal conductivity measured for grain sizes of approximately 100 nm were found to be at least a factor of two lower than those reported for single crystal samples. The thermal conductivity of TiTe2 ranged between 0.15 W m-1 K-1 to 0.21 W m-1 K-1, consistent with turbostratic disorder between the Te-Ti-Te planes of this layered compound. The thermal conductivity of [(Bi2Te3)]m[(TiTe2)1.36] n superlattices increased systematically as the bismuth telluride content was increased. Preparing (BixSb1-x)2Te 3 or [(BixSb1-x)2Te3)] m[(TiTe2)1.36]n alloys further reduced the thermal conductivity relative to the pure binary compounds or [(Bi 2Te3)]m[(TiTe2)1.36 ]n, suggesting that alloying either additionally scatters the same phonons or scatters different heat carrying phonons. A minimum in the thermal conductivity was observed at a 50-50 bismuth to antimony ratio, with values as low as 0.16 W m-1 K-1 measured for [(Bi0.5Sb0.5)2Te3)]m [(TiTe2)1.36]n compounds.Typically any reduction in thermal conductivity is offset by reductions in the electrical transport properties. Successful preparation of small grained materials having high electrical conductivity and Seebeck coefficients was achieved by controlling the defect levels in the material and avoiding high resistance oxide coatings on the grains. Control over the carrier concentration was achieved by constructing an annealing system where materials can be annealed under an equilibrium vapor pressure of chalcogenide.The findings of this dissertation were enabled by pushing the limits of the synthetic technique used to prepare the materials of study. Compositions could be reproducibly prepared with variations of less than 1% from sample to sample. Thickness control of the nanostructure was to within a tenth of an Angstrom. More complex structures made by interleaving 3 components in an ABAC pattern were prepared with precise control over the thickness and composition.This dissertation includes co-authored material that has not been published previously.